The present invention relates to hygrometers, more specifically to chilled mirror hygrometers.
Many methods of humidity determination are based upon inferring the moisture content of a sample from an output of a moisture sensitive transducer. Such secondary methods may be subject to such inherent limitations as a non-linear variation of the output parameter with humidity, drift, temperature sensitivity, hysteresis and aging. The limitation of temperature, of course, relates to the fact that water holding capacity of a gas varies with temperature so that only a relative humitidy rather than an absolute measurement is provided. Relative humidity is related to absolute humidity by the Goff-Gratch equation, which is not conveniently embodied in a linear circuit.
Optical chilled mirror hygrometers are preferable since they provide an indication of dew point or frost point, each of which is a primary measurement of moisture content.
The dew point is the temperature at which the partial pressure of a condensate on a surface equals the water vapor partial pressure in a gas. Similarly, frost point is the saturation temperature to which the gas temperature must be cooled at constant pressure so that it will be saturated with respect to ice. Saturation vapor pressure is a unique function of temperature. Therefore, determining the temperature at which water vapor begins to condense on a cool surface is equivalent to a measurement of its partial pressure.
Optical chilled mirror hygrometers use cooled mirrors as the surface where condensation takes place. Utilizing electrooptic circuitry, a precise determination of the formation of either dew or frost is made. A temperature transducer, for example a platinum resistance transducer, is used to provide the signal indicating the output information.
In the past, a limitation of optical chilled mirror hygrometers has been the inability to differentiate between dew or frost and dirt deposits since both will be measured by a photosensor as a loss of reflected light. A prior art response has been to establish a mode of utilization of such hygrometers in which a cleaning procedure is performed periodically or on an event-dependent basis, e.g. such as at the beginning of a day's operation. Another compensating procedure is the "balancing" of a system. This is done by in essence performing a calibration operation to "null" variations in the optic sensing scheme due to dirt deposits on the mirror surface. A balancing operation calls for heating of a mirror to a temperature higher than the dew point to insure a dry mirror so that loss in reflectivity due to dirt alone may be measured. Adjustment of optical sensing circuitry must be made to compensate for any differences to respect to original calibration values.
In both situations, cleaning or calibration must be performed more often than would be called for due to any deterioration in performance so that such deterioration in performance, which would be otherwise undetected, can be avoided. Furthermore, balancing or cleaning operations necessarily require losing the ability to monitor while the operations are being performed. The operations are performed without reference to the performance of the instrument. No way is provided apart from the actual cleaning or balancing operations to increase the time between which "down" periods of the hygrometer must occur.